Currently, various types of multifunctional epoxy compounds (hereinafter, also referred to as epoxy resins) each having two or more alicyclic skeletons in the molecule are commercially available. Examples thereof include: 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate (e.g., CELLOXIDE 2021 manufactured by Daicel Chemical Industries, Ltd., ERL4221 manufactured by Union Carbide Corporation, etc.); 1,2,8,9-diepoxylimonene (e.g., CELLOXIDE 3000 manufactured by Daicel Chemical Industries, Ltd.); one (e.g., CELLOXIDE 2081 manufactured by Daicel Chemical Industries, Ltd.) in which 3,4-epoxycyclohexylmethanol and 3,4-epoxycyclohexane carboxylic acid are coupled with both ends of an ε-caprolactone oligomer through ester linkages as disclosed in each of JP 4-36263 A and JP 4-170411 A; and bis(3,4-epoxycyclohexylmethyl)adipate (e.g., ERL4299 manufactured by Union Carbide Corporation). Alternatively, epoxidized 3-cyclohexene-1,2-dicarboxylate bis-3-cyclohexenylmethyl ester and an ε-caprolactone adduct thereof (GT300 series such as “EPOLEAD GT301” manufactured by Daicel Chemical Industries, Ltd.) disclosed in JP 4-69360 A and JP 4-170411 A, and epoxidized butane tetracarboxylate tetraxis-3-cyclohexenylmethyl ester and an ε-caprolactone adduct thereof (GT400 series such as “EPOLEAD GT401” manufactured by Daicel Chemical Industries, Ltd.) are also commercially available as curable epoxy compounds each having a plurality of alicyclic epoxy groups. Cured products can be obtained by allowing such multifunctional epoxy compounds to react with various curing agents and curing catalysts. A cured product of an epoxy resin is allowed to have good heat-resistance, transparency, and dielectric properties that are characteristics of a resin prepared from a compound with an alicyclic skeleton. Such an epoxy compound is useful as an ingredient to be included in coatings, adhesives, ink, and sealants, or as an intermediate to prepare any of other valuable compounds in a variety of end uses including pharmaceutical agents and medical supplies.
CELLOXIDE 3000 has a methyl group on a carbon atom in the epoxy group so that the reactivity thereof is low due to its steric hindrance. Alternatively, since CELLOXIDE 2021, CELLOXIDE 2081, or ERL4299 has an ester group in the molecule, it has hydrolyzability. If they are used under high temperatures and high moistures or under such a condition that a strong acid occurs, cured products thereof may suffer from decrease in physical properties.
Thus, a multifunctional epoxy compound with an alicyclic skeleton having no ester group in the molecule has been desired.
The Russian literature (Neftekhimiya, 1972, 12, 353.) discloses dicyclohexyl-3,3′-diepoxide as the representative of alicyclic diepoxy compounds represented by the general formula (I) described below. In the literature, peroxydate (where the peroxydate refers to t-butylhydroperoxide) as an epoxidizing agent and molybdenum chloride (V) in a catalytic amount are utilized for synthesis. In this literature, the peroxydate is used at high temperatures above 80° C. or more. Therefore, as the risk that the peroxydate is explosively decomposed is involved, there is a problem in safeness. In addition, molybdenum chloride (V) used as a catalyst is expensive and strongly poisonous. Therefore, a preparation process that is economical and gives environmental consideration has been sought.
On the other hand, a cured product of an epoxy resin typically has excellent performance in mechanical properties, water resistance, corrosion resistance, adhesion, chemical resistance, heat resistance, electrical characteristics, and so on, and is therefore utilized in a wide range of fields such as adhesives, paintings, laminated boards, encapsulating materials for IC, and molding materials.
Of those, for example, aromatic epoxy resins for general purposes, a glycidyl ether-based epoxy resin typified by a bisphenol-based epoxy resin, a phenol novolak-based epoxy resin, or the like, is used as a cured product to be cured under various curing conditions by the addition of a curing agent and optionally a curing accelerator, and, if necessary, the addition of a filler such as talc, titanium, or silica.
However, the cured product composed of the above aromatic epoxy resin for general purpose that is a glycidyl ether-based epoxy resin has the structure of an aromatic nucleus, so it has inferior weatherability outdoors. Additionally, when the viscosity of the glycidyl ether-based epoxy resin described above is measured at 25° C. using an E-type rotation viscometer (e.g., one manufactured by Tokyo Keiki Co.), the glycidyl ether-based epoxy resin typically has low fluidity, for example, 4,000-20,000 mPa·s for a bisphenol A type and 1,500-4,500 mPa·s for a bisphenol F type. Therefore, the glycidyl ether-based epoxy resin is mostly used by being dissolved in a solvent typified by toluene, xylene, methyl ethyl ketone, ethyl acetate, or the like, which causes a problem with workability and environmental safeness.
Known examples of an epoxy resin that has sufficiently low viscosity even if no diluent is used include one having a cyclohexene oxide skeleton (alicyclic skeleton). An epoxy compound having an alicyclic skeleton is characterized by having the same degree of reactivity as that of a glycidyl ether-based epoxy compound and is currently commercially available in various types. Examples of a monofunctional epoxy compound with an alicyclic skeleton in the molecule include monoepoxidized 4-vinylcyclohexene. Examples of a bifunctional epoxy compound include 4-vinylcyclohexene diepoxide and limonene diepoxide, and the like.
Such the compound having an alicyclic skeleton is free of halogens because no halide is used in its preparation process, and thus has superior electrical characteristics. Moreover, the compound is allowed to have heat resistance and transparency that are characteristics of a resin using a compound with an alicyclic skeleton.
Such the epoxy compound with an alicyclic skeleton and a resin composition containing it are used in the ingredients of coatings, adhesives, ink, and sealants, or stabilizers for various thermoplastic resins, or in a variety of end uses including pharmaceutical agents and medical supplies. In addition, it is known that the epoxy compound and the resin composition are useful also as intermediates for preparing other valuable compounds.
Since the epoxy compound with an alicyclic skeleton has sufficient performance but slightly lower reactivity when used in the above applications, the cured product thereof may suffer from decrease in physical properties and reactivity. Thus, a highly reactive alicyclic epoxy compound has been desired. Furthermore, the foregoing monoepoxidized 4-vinylcyclohexene and limonene diepoxide evaporate at room temperature, which causes a problem with working surroundings.
Alternatively, one example of an alicyclic diepoxy compound similar to alicyclic diepoxy compounds represented by the general formula (I) according to the present invention described below is an alicyclic diepoxy compound in which two alicyclic structures are connected by a methylene group or the like (e.g., JP 58-172387 A and JP 50-10636 B).
Additionally, in recent years, a curable resin composition obtained by blending or modifying an oxetane compound, a cationic polymerization initiator, and an alicyclic diepoxy compound has been proposed (e.g., JP2002-53659 A and JP2002-82527 A). However, none of those curable resin compositions each including the alicyclic diepoxy compound has displayed satisfactory performance yet.
The inventors of the present invention have found that a curable epoxy resin composition containing: an alicyclic diepoxy compound represented by the general formula (I) described hereinafter; and an initiator for thermal cationic or photocationic polymerization, or an acid anhydride, and a cured product obtained by curing the composition have excellent properties.
Conventionally, electronic parts such as diodes, transistors, and integrated circuits are encapsulated with thermocuring resins. Specifically, in an integrated circuit, systems each composed of an o-cresol novolak-based epoxy resin or biphenyl-based epoxy resin system and a novolak-based phenol resin that have excellent heat resistance and moisture resistance are mostly used.
In pursuit of miniaturization, weight reduction, and enhanced performance of recent electronic equipments, along with the integration of semiconductors, a conventional through-hole mounting manner where a lead pin is inserted into a hole of a substrate is being replaced with a surface mounting manner where components are soldered to the surface of a substrate. In the surface mounting manner, unlike the through-hole mounting manner, the entire package sealed with an encapsulating resin is heated at high temperatures of 210 to 270° C. at the time of soldering during the mounting process. Therefore, cracks occur in the portion of the resin and cracks and exfoliation occur around a chip, which reduces reliability, resulting in a problem of unusability as a product.
Although many hypotheses are put forward regarding the mechanism of the emergence of cracks, the mechanism is generally considered as follows: during the mounting process, the encapsulating resin for a package absorbs moisture; on the other hand, in the surface mounting operation, the entire package is exposed under high temperatures above 200° C., and, in the case of a thin package, the temperature within the package exceeds 200° C. in a short period of time; if the package absorbing moisture is rapidly heated above 200° C., an internal pressure is generated by the vaporization of the moisture; when the internal pressure of the package exceeds the rapture strength of encapsulating materials, cracks occur.
Moreover, boundary separation may be attributed to the contraction in volume that take places during the curing of a curable resin such as an epoxy resin, or to the thermal stress within the package that is generated by the difference of the coefficients of linear expansion between a metal and a molding material for the epoxy resin.
In order to overcome the above disadvantages, a method of increasing the elasticity of the resin for encapsulation or decreasing the coefficient of liner expansion has been employed. One example is a method for higher loading of a filler. Although higher loading of a filler is effective means, it has limitations and may cause increase in viscosity and decrease in fluidity of the resin for encapsulation. The increased viscosity of the resin for encapsulation results in the deformation and cutting of a lead wire and the decreased fluidity results in lower loading, which may reduce reliability.
A variety of multifunctional epoxy compounds each having an alicyclic skeleton in the molecule have been known as resins for the encapsulation of electronic parts.
However, as described above, 1,2,8,9-diepoxylimonene has a methyl group in carbon constituting an epoxy group and thus the epoxy group has low reactivity compared to one having no methyl group. In addition, 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexane carboxylate, bis(3,4-epoxycyclohexylmethyl)adipate and a lactone adduct thereof, and epoxidized butanetetracarboxylic acid tetraxis-3-cyclohexenylmethyl ester and an ε-caprolactone adduct thereof each have an ester group in the molecule so that hydrolysis may be generated.
Therefore, as described above, the physical properties of the cured product may be lowered owing to the use under high temperatures and high moisture and to hydrolysis.
In addition, as described above, an alicyclic epoxy compound in which two alicyclic structures are connected by a methylene group or the like has been known as an alicyclic epoxy compound similar to the alicyclic epoxy compound (a) represented by the above formula (I) (e.g., JP 2002-275169 A, JP 58-172387 A, and JP 50-10636 B). However, the epoxidation rate of the epoxy compound is somewhat low.
In addition, JP 2001-181481 A discloses an epoxy resin composition for the encapsulation of semiconductors utilizing a biphenyl-based epoxy resin, which has superior water-absorbing properties but still has a problem with fluidity and torque in curing, that is, moldability.
Thus, it is desired to develop a highly flowable epoxy compound having another alicyclic skeleton without an ester group in the molecule. The inventors of the present invention have found that an epoxy resin composition containing: the alicyclic epoxy compound (a) represented by the general formula (I) described below; and a curing agent has excellent properties as a resin composition for the encapsulation of electronic parts.
In an application for an electrical insulating oil, an electrical insulating oil for oil-immersed transformers, specifically open-type transformers, in which the insulating oil contacts air, must be excellent in oxidation stability. Thus, one containing an antioxidant, particularly a phenolic antioxidant is studied (JP 9-272891 A). However, for example, an electrical insulating oil (JP 2002-260445 A) supplemented with 0.3% by weight of DBPC (2,6-di-tert-butyl-p-cresol) shows that a total acid value already rises (0.02-0.03 mg KOH/g) through the phenomenon of oxidation at 120° C.×75 hours in the oxidation stability test of JIS C 2101-1993. That is, it is thought that the induction period of oxidation terminates in 75 hours and then a rise in total acid value proceeds abruptly.
On the other hand, as a technique for using an epoxy compound as a stabilizer for an electrical insulating oil to entrap impurities in the component of the insulating oil and to disperse discharge energy, an insulating oil for capacitors has been disclosed in JP 3-171510 A and JP 7-226332 A. However, the epoxy compound utilized is mainly an alicyclic diepoxy compound or an aromatic diepoxy compound having, in the molecule, an ester linkage or an ether linkage which generates hydrolysis and pyrolysis in the long-term use and may not occasionally show an antioxidant function as a result. Moreover, in general, since a capacitor has the sealed structure in which an insulating oil is enclosed, it is deformed by gas generated from decomposition or made unusable owing to the reduction in breakdown voltage and capacitance.
On the other hand, in an insulating portion of electric equipment, a connecting portion of a power cable, and the like, a cast article of an epoxy resin composition embedded with a metal electrode is arranged so as to support a conductor in the portion of the above metal electrode. In general, such a cast article of the epoxy resin composition is produced using an epoxy resin composition composed of a multifunctional epoxy resin, an acid anhydride, a filler, and so on. In particular, a bisphenol-based epoxy resin as the epoxy resin, phthalic anhydride as the acid anhydride, and inorganic powder such as alumina or silica as the inorganic filler are used. Crack resistance, mechanical strength, and electric properties have been balancedly improved by using them.
Recently, the tendency for the miniaturization in size and ultra high-voltage of a high-voltage instrument is intensifying more and more. The cast article of the epoxy resin composition having further sophisticated performance is required. Thus, a conventional cast article of an epoxy resin composition has limitations on electric properties and mechanical properties and potentially leads to breakdown. That is, along with a shift to an ultra high-voltage, an insulator (using the casting epoxy resin composition for electrical insulation) is subjected to a high electric field. Therefore, the insulator is required to further improve withstand voltage strength. Moreover, since dielectric loss (ε.tan δ.E2) is enlarged along with an ultra high-voltage, the thermal damage of an insulator may be caused by the generated heat. Specifically, if ε.tan δ increases with increasing temperature, the breakdown by thermal runaway cannot be denied. Additionally, the adhesiveness with a metal electrode embedded in an insulator and the crack resistance must be further improved.
For example, as disclosed in JP 9-77847 A, by using a bisphenol A-based epoxy resin and a novolak epoxy resin as epoxy resins, a resin for insulation is made by mixing two or more acid anhydrides. Similarly, JP 11-60908 A illustrates that a resin for insulation is made by mixing a bisphenol A-based epoxide and a crystalline epoxide as epoxy resins with two or more acid anhydrides.
Those compositions are not suitable for casting and impregnation because the compositions are of solid.